In the mining, processing and handling of coal enormous tonnages of fines are created. Typically, about fifteen to twenty percent of the tonnage mined, after handling and cleaning is completed, comprises fines having a size range from powder up to small granular.
Much of this coal fines is not directly usable, and historically the problem has only recently been reapproached, due to the resurgence of coal as a consequence of the politicised escalation of oil prices, as a result of increasingly more stringent customer demands in respect of the quality of coal supplied them by the producers, and because of the increasing regulation of mine waste disposal practices to satisfy environment standards.
The prior art vis a vis coal briquetting focused on the low pressure briquetting of coal fines, using a binder, typically of coal tar origin, to hold the individual particles together. While this technology flourished during the early part of the present century, when the binder-briquetted product was substantially utilized as a home heating fuel, this application has essentially disappeared since the end of World War II as a result of a shift to other and more convenient sources of fuel.
While a certain portion of the fines can often be sold to the customer in combination with the coarser fractions of the mine production, the inclusion of the whole of the recoverable quantities of fines may result in downgrading the quality of the combined product below market requirements.
The fines material is frequently in the form of a wet filter cake, containing between about twenty and thirty percent moisture, depending upon its size distribution and ash content. In a dry state however, the fines are generally predominantly passable through a twenty eight mesh screen. Up to 1/16 inch mesh screen may be used.
Another potentially available form of high quality fines is the discard material produced by current and/or previous coal preparation facilities in which the fines were not efficiently recovered by the previous process, or else were present in such quantities as to make their total recovery and incorporation into the mine product impractical in respect of market requirements.
In the recent past the reconstitution of fines by extrusion, pelletizing and briquetting generally has involved the use of a binder or binders, including starch, sodium chloride, portland cement, and coal tar, now recognized as a carcinogen.
In use it has been found that medium to small sized briquettes of regular form provide excellent combustion characteristics on chain grates and similar types of stoker arrangements. However, the presence of the binders can contribute to highly undesirable air pollution, and/or give rise to undesirable combustion characteristics in the reconstituted fuel.
Also, the use of binders adds significantly to the cost of the reconstituted product.
Attempts to carry out binderless briquetting at low temperatures have encountered problems, both in the handling of the fines feed stock, and also in the strength and other physical characteristics such as water absorbtion of the product thus produced.
Many aspects of the prior art handling of coal fines are dealt with in the following listed publications:
U.S. Department of Energy Pubn. ET 14303 Oct. 1981: "Briquetting Of Fine Coal Using A Sodium Chloride Binder". PA0 British Coal Board: Paper by G. S. Jones & D. B. Meecham: "The Pelletization of Fine Coal"--Parts I and II. PA0 Battelle Laboratories, Columbus, Ohio, U.S.A. Report by W. H. Conkle and J. W. Dawson: "Reconstitution of Physically Cleaned Coal."
Other attempts to briquette coal fines have involved medium pressure, long duration binderless processes wherein the coal fines require a preparatory treatment of washing in an organic solvent medium, in order to liberate material from the coal, to serve as a binder in the subsequent pressure forming process. The press pressures used range from 4,000 to 30,000 p.s.i., and the process appears to yield an improved product. However, solvent recovery presents capital cost and other problems, to render the process impractical. This work is to be found in the Iowa State University technical paper IS-ICP-67 for the Energy and Mineral Resources Research Institute: "Coal Briquetting Without a Binder," Miller et al Oct. 1, 1979; and in U.S. Pat. No. 4,235,603 Nov. 25, 1980, Miller et al.
In considering the significance of the physical characteristics of reconstituted coal fines in briquette form it should be born in mind that commercial use generally invovles bulk handling with storage in a stockpile open to the elements, and bulk transfer, using heavy machinery. Thus, in addition to the abuses of mechanical handling, and the usual self weight loading which occurs in a high stockpile, the effect of the elements leads to repetitious wetting and drying of the briquettes.
In the case of porous or semi-porous briquettes which can readily take on water, in addition to sustaining physical damage and suffering an effective calorific heat loss due to any absorbed water load, the batch heating value of the porous briquettes of the prior art is unpredictable, while the stockpile itself may become a source of environmental polution.